1. Field of the Invention
The present invention relates to stems for an optical element and optical semiconductor devices using the same, and more particularly to a stem for an optical element to which an optical element for receiving/emitting a high-frequency signal used in optical communication or the like is mounted, and an optical semiconductor device using the same.
2. Description of the Related Art
In recent years, the broadband promotion in the communication has progressed and the public communication network using optical fibers has been spread. Along with these circumstances, it has been required more and more to inexpensively transmit a large quantity of information. In order to increase a quantity of information to be transmitted so as to meet such a demand, it is necessary to increase a transmission rate. Thus, the transmission rate has been progressively increased from 600 Mbps to 2.5 Gbps, and up to 10 Gbps.
Networks of the optical communication as a market have been spread not only to a main line system, but also to an access system such as an office or a home with an increase in transmission speed of such a device for optical communication as the background. Thus, high-speed operation, inexpensiveness and high efficiency are required for a light emitting/receiving device used in an optical transmission receiver.
Many optical devices use an inexpensive can package that has a coaxial shape in order to hold the cost inexpensive. Then, a package construction having a less propagation loss is required for such a can package as well.
A main body of the can package is called a stem. The stem includes a metallic disc called an eyelet, a several rod-like lead electrodes, and a heat sink called a block. The rod-like lead electrodes are sealed in a plurality of through holes provided in the eyelet with sealing glass and are adapted to propagate an electrical signal therethrough. The heat sink is mounted onto the disk surface of the eyelet and an optical element is bonded to the heat sink.
In order to reduce the high-frequency propagation loss as much as possible, it is necessary to connect the optical element and the lead electrodes with lead wires as short as possible. For this reason, a sub-mount is provided on one side face of the block, and the optical element is bonded to the vicinity of a central portion of the sub-mount. In addition, signal lines are provided on the sub-mount on both sides or one side of the optical element. Electrodes of the optical element are connected to the signal lines short lead wires, respectively, and also the signal lines are connected to the lead electrodes with short lead wires, respectively. As a result, the high-frequency propagation loss in the connection between the optical element and the lead electrodes is reduced as much as possible.
In terms of the high-frequency propagation loss when the signal is propagated through the lead electrodes, the high-frequency propagation loss in the glass sealing portions between the through holes of the eyelet and the lead electrodes is reduced as much as possible by properly performing the impedance matching in the glass sealing portions. However, the impedance matching is hardly properly performed in portions, of the lead electrodes, which project from the eyelet in some cases.
The lead electrodes and the block as GND are separated from each other through air having a small dielectric constant. Hence, in order to reduce the high-frequency propagation loss in the projecting portions, of the lead electrodes, which project from the surface of the eyelet as much as possible, it is necessary that the lead electrodes and the block be provided so as not to come into contact with each other, but so as to come close to each other as much as possible.
However, in order to reduce the high-frequency propagation loss in the lead electrodes and the eyelet, the sealing glass to be provided in the through holes are injected into the through holes to be solidified until it becomes flush with the disc surface of the eyelet. At this time, the liquid sealing glass is drawn up in its portions contacting the surfaces of the lead electrodes due to its surface tension, forms meniscuses depending on the wettability of the material of the lead electrodes, and is solidified while holding the meniscuses.
For this reason, even if being made close to the lead electrodes, the block interferes with the projecting portions of the solidified sealing glass, formed due to the meniscuses of the sealing glass in the peripheries of the lead electrodes, and a distance between the block and the lead electrodes cannot be sufficiently reduced in some cases. Thus, the high-frequency propagation loss increases in some cases as the transmission rate of the signal increases.
Such methods of known examples are the following. That is to say, the impedance in the lead electrode and the block is made close to a given value, and the interference between the block and the projecting portion formed due to the meniscus of the sealing glass with which the lead electrode is sealed is avoided.
One of the known examples is as below. A coaxial hole of a block through which a lead electrode is made to completely extend is formed so as to have a smaller diameter than that of a through hole of an eyelet. In addition, in a portion where the coaxial hole of the block and the through hole of the eyelet are coupled to each other, an inner surface of the coaxial hole of the block is formed into a tapered surface in order to avoid interference with a projecting portion formed due to a meniscus. Refer to Japanese Patent Application Laid-Open No. 2004-127963, paragraphs [0009] to [0013], and FIGS. 1, 2 and 3 for example.
In addition, another known example is the following example of a coaxial line for a package. That is to say, a through hole is bored through a metal wall corresponding to an eyelet. The through hole has an outer end with a larger diameter than that of its inner end and a lead is inserted through the through hole. In addition, a sealing plate for covering an opening portion, of the through hole inner end, opening in a boundary portion between the through hole inner end and the through hole outer end is provided in the boundary portion therebetween, and the sealing plate is sealed in the through hole outer end with glass. Thus, the glass is prevented from being stuck to a lead peripheral surface. Refer to Japanese Patent Application Laid-Open No. 6-29451, paragraphs [0035] to [0042], and FIG. 2 for example.
Moreover, still another known example is disclosed in which a stem block having a recess stepped portion below a surface that a semiconductor laser diode is fixed is formed integrally with the stem base, and the recess having such a size that at least a part of a photodiode fixed to the stem base intrudes thereto. Refer to Japanese Patent Application Laid-Open No. 8-125259 for example.
Furthermore, yet another known example is disclosed in which signal pins mounted to a stem with a dielectric body is brazed to strip differential signal lines on a micro-strip differential line substrate provided on a surface of the stem. In the case of this example, a surface of the dielectric material with which the signal pins are mounted is formed in a position lower than that of the surface of the stem. Refer to Japanese Patent Application Laid-Open No. 2004-47830, paragraphs [0049] to [0051], and FIGS. 5 and 6 for example.
However, for the purpose of providing an inexpensive stem for an optical element, it is very important to simplify the manufacturing process thereof. The technique as shown in one of the above-mentioned known examples in which for example, the coaxial holes through which the lead electrodes are made to extend completely are provided in the block mounted on the eyelet, the distance to the lead electrodes is precisely ensured, and moreover the block having the complicated shape is formed such that the inner surface of the coaxial hole is formed into the tapered surface leads to an increase in manufacturing cost.
Likewise, the technique in which the stepped holes having the different inside diameters are formed, and the sealing plate is provided in the boundary portion between the through hole inner end and the through hole outer end, or the stem block having the recess stopped portion is formed integrally with the stem base also leads to an increase in manufacturing cost. Then, there is encountered such a problem that it becomes difficult to provide an inexpensive stem for an optical element in some cases.